c++ - implementing a non blocking udp socket by select() -

i wanted create asynchronous/non-blocking udp client server application client , server supposed chat away each other without waiting each other's turns. came know done select()...

here server(mentioning communication part):

fd_set readfds,writefds;  while(1){     fd_zero(&readfds);     fd_zero(&writefds);     fd_set(sd,&readfds);     fd_set(sd,&writefds);      int rv = select(n, &readfds, null, null, null);     if(rv==-1)      {         printf("error in select!!!\n");         exit(0);     }     if(rv==0)      {         printf("timeout occurred\n");     }     if (fd_isset(sd, &readfds))     {              int client_length = (int)sizeof(struct sockaddr_in);         memset(&buffer,0,sizeof(buffer));         int bytes_received = recvfrom(sd, buffer,size, 0, (struct sockaddr *)&client, &client_length);         if (bytes_received < 0) {             fprintf(stderr, "could not receive datagram.\n");             closesocket(sd);             wsacleanup();             exit(0);         }     }      printf("\nclient says: %s",buffer);     printf("\nwrite :");     fgets(buffer,size,stdin);      if(fd_isset(sd,&writefds)) {         int client_length = (int)sizeof(struct sockaddr_in);         if(sendto(sd, buffer,strlen(buffer), 0, (struct sockaddr *) &client,client_length)<0) {             printf("error sending file! \n");             printf("%d\n",wsagetlasterror());             exit(1);         }     } } closesocket(sd); wsacleanup();  return 0; 

}

and client:

fd_set readfds,writefds; while(1) {     fd_zero(&readfds);     fd_zero(&writefds);     fd_set(cs,&readfds);     fd_set(cs,&writefds);     int rv=select(n,&readfds,&writefds,null,null);             if(rv==-1)     {         printf("error in select!!!\n");         exit(0);     }     if(rv==0)     {         printf("timeout occurred\n");     }     printf("\nwrite  ");      fgets(send_buffer,size,stdin);     if(fd_isset(cs,&writefds))     {         int server_length = sizeof(struct sockaddr_in);         fd_clr(cs,&writefds);          if (sendto(cs, send_buffer, (int)strlen(send_buffer) + 1, 0, (struct sockaddr *)&server, server_length) == -1)         {             fprintf(stderr, "error transmitting data.\n");             closesocket(cs);             wsacleanup();             exit(0);         }     }      char file_buffer[size];     //reply reception server:"ready receive file"     int data2=0;     if (fd_isset(cs, &readfds))     {         fd_clr(cs,&readfds);         int server_length = sizeof(struct sockaddr_in);         data2=recvfrom(cs,file_buffer,strlen(file_buffer)-1,0,(struct sockaddr *)&server,&server_length);         //file_buffer[data2]='\0';         if(data2<0)         {             printf("server not on:(\n");             exit(0);         }     }     //printf("%d",data2);     printf("\nserver says:");     for(int i=0;i<data2;i++)     {         putchar(file_buffer[i]);     }  }  return 0; } 

at first on server side wrote:int rv = select(n, &readfds, &writefds, null, null);

but led printing of entire empty array on server console when server initialised in addition fact communication between server , client not proper. removing "&writefds" did remove redundant data improper communication issue still persists... thankful if helped me out...

i see couple of problems code.

first of all, if want wait input both socket , terminal, should put both of fds readfds set:

fd_set(cs, &readfds); fd_set(stdin, &readfds); 

since you're not doing writing, shouldn't using writefds.

next, should make sure you're trying read terminal if has input ready. so, should doing like:

if (fd_isset(stdin, &readfds)) {     // read in terminal... } 

you're trying read each time, no matter happens.

last, you're measuring size of file_buffer incorrectly when make recvfrom call. strlen works once you've put data it. should using sizeof(file_buffer).